wireless not working correctly after tea time

I have a netgear router and have been using it for over a year now, no problem. For the past few days when I try connecting wirelessly to the net or my son tries with his phone, it says that it cannot find our connection. We look in the list of available networks and ours is not there amongst the four that usually appear, but we have noticed that since the beginning of the week 3 new networks have been picked up and 1 is from Virgin (as is ours) and 2 are from bt (ours is the 802.11g) and the 3 new ones are 802.11n. I am wondering could these be affecting our signal? because during the day they are not listed and all is ok but they appear from early evening onwards and that is when we have problems connecting.
Sorry it's a long question but wanted to get all info on.
Thanks Ian.
PS there is no problem when connected to router via cable as the wife uses that and all is ok she said.

There may be a conflict with neighbouring wireless configurations. You could have a look at changing the wifi frequency of your router - whatever it is on , 1, 6, 11 seem to be recommended. To access your router you will need to type into the address bar at the top of your browser the IP address of your ISP - usually 192.168.1.1 or 1.2 or something similar. You will then have to put in your user name and password - if you haven't changed them they will probabably be admin/admin, or admin/password or admin/blank. Go into settings and find wifi channel.

Try inSSIDer
rather than Netstumbler, it is not restricted to 802.11a,b or g networks. Don't worry about using only channels 1, 6 or 11, use whichever channel is furthest from those in use by neighbours for least interference.

Just a thought - do you have a DigiSender attached to your TV which is switched on after teatime? I know this is why our Wireless connection stopped working - once the DigiSender is switched off, hey presto, the wireless signal is OK. Took a long time to discover what the problem was; after the ISP provider had investigated etc. we found out ourselves.

thanks to all who have replied with suggestions. In response to the last two I do not have a digisender on the tv as we are with Virgin and have had the same set up for years, also the wifi is not and never has been set to shut down/turn off at specific times (if it was then the problem would have arisen a long time ago). Like i said the problem has arisen since 3 new wi fi connections were discovered by my pc, all of which are BT owned, I complained to BT who told ME to readjust/set my wifi settings, my reply was why should I when it has been running ok for years? I got in touch with Virgin who sympathised and they sent out a new router to me and all (so far) is now running smoothly. Again thanks for all the replies
Kind regards Ian